1. Field of the Invention
The present invention relates to a display device. The invention particularly relates to a structure of a pixel electrode in a display device.
2. Description of the Related Art
In recent years, a light emitting element using a light emitting organic compound has been actively researched and developed as a type of light emitting element. A typical light emitting element has a layer containing a light emitting organic compound (light emitting layer) interposed between a pair of electrodes. An electron and a hole are respectively injected and transported from each of electrodes to the light emitting layer by applying voltage to the element. The light emitting organic compound is excited by recombination of those carriers (an electron and a hole) and emits light in returning from the excited state to a ground state.
Such a light emitting element can be formed with a thin film having a thickness of submicrometer to several micrometers. Since light is emitted in only microseconds or less after carriers are injected, the response speed is extremely high, which is a feature of such a light emitting element. Further, enough light can be obtained with direct current voltage of several volts to several tens of volts; thus, the power consumption is also low in comparison. Because of such advantages, the above light emitting element attracts attention as a next generation flat panel display element.
In such a light emitting element, since a pair of electrodes and a light emitting layer are formed to a film shape, a large area element can be formed; thus, planar light emission can easily be realized. Since this feature is difficult to obtain with a light source such as an incandescent lamp or an LED (point source), or a fluorescent lamp (linear light source); therefore, the above light emitting element is also expected to be used as a light source of a lighting.
Further, as a method for displaying an image on a display device, a passive matrix type and an active matrix type, where arbitrary display is performed by selecting lighting or non-lighting of each pixel in a display area including a plurality of pixels, are generally used. As to the passive matrix type, wirings are provided in a reticular pattern, and lighting or non-lighting of the area where the wirings in columns and wirings in rows cross is controlled, thereby displaying an image. As to the active matrix type, a thin film transistor (TFT) is provided in each pixel of a display device, and the pixel is controlled to emit light or not using the TFT as a switching element thereby displaying an image. The active matrix type can achieve higher contrast and lower power consumption drive than the passive matrix type; thus, it is applied to more display devices.
However, in an active matrix display device, for example, in the case where a failure due to damage, defect, or the like of a TFT provided in each pixel is caused, current or voltage to be supplied to the pixel becomes uncontrollable. As a result, high current is constantly supplied to a light emitting layer provided in the pixel; thus, the pixel could constantly emit light. Such a state where a pixel constantly emits light leads to a defect (bright pixel defect) where the pixel constantly emits light in displaying an image; thus, the salability of the display device significantly suffers.
Further, in a display device using a light emitting element, a light emitting layer is formed to be very thin; thus, a defect such as a pinhole can easily be caused in a part of the light emitting layer. When a defect is caused in a light emitting layer, short circuit is caused between an anode and a cathode and current is concentrated onto the defect portion; thus, high current flows into a pixel including the defect. Consequently, problems such as effects on surrounding pixels associated with increase in power consumption or increase in temperature of the defect portion are caused.